Traditional methods of imaging (or printing) use various types of long-run print forms, such as gravure cylinders, offset plates and flexographic belts, which carry a recorded representation of a desired image (or “signature”). For example, lithographic offset printing methods typically use aluminum plates carrying imagewise signatures on rasterized ink-accepting and ink-repellant areas. A lithographic offset plate usually is imaged by applying an ultraviolet contact photography process to a sheet of silver film. In this process, exposed raster dot areas are etched from an initial ink-accepting state into a water-accepting state; unexposed raster dot areas remain in an ink-accepting state. Lithographic inks are typically hydrophobic, exhibit high viscosities and contain small amounts of solvent.
Other imaging methods, such as marking methods, do not require printing forms. For example, ink jet printing produces images by ballistically jetting a serial sequence of ink droplets from a distance onto a substrate (e.g., a paper sheet). Ink jet printing inks generally are volatile, exhibit low viscosity, and may be loaded into an ink jet printer in a liquid or a solid state. Some solid ink jet inks may be activated by heating. Other solid ink jet inks, such as inks containing rheological fluids, may be activated in other ways. Magneto-rheological fluids are responsive to magnetic fields, whereas electro-rheological fluids are responsive to electric fields. One system has proposed an ink composition that is suitable for use in ink jet printing and includes a coloring agent and a carrier containing a magneto-rheological fluid with viscosity and flow properties that may be controlled by an applied magnetic field. Another system has proposed an ink jet ink composition that includes an electro-rheological fluid that enables the ejection of ink to be controlled by applying an electric field that varies the viscosity of the ink and by creating a pressure difference in a venturi tube.
Electrostatic printing methods also do not require printing forms. In these methods, a discharge source typically deposits imagewise electrostatic charges onto a dielectric member (e.g., a plate or a drum) to generate an electrostatic latent image on the dielectric member. The latent image is developed into a visible image by depositing a charged developing material onto the surface of the dielectric member. Charged solids in the developing material adhere to image areas of the latent image. The developing material typically includes carrier granules having charged marking or toner solids that are electrostatically attracted from the carrier granules to the latent image areas to create a powder toner image on the dielectric member. In another electrostatic imaging method, an electrostatic latent image is formed directly in a layer of toner material as opposed to on a dielectric member.
In this method, an image separator is electrically biased to selectively attract either image or non-image areas of the latent image formed in the toner layer. In one process, latent images are formed by electrocoagulation of an ink composition. In particular, the electrocoagulation involves an electrochemical reaction that affects an electrolytically sensitized polymeric ink. In this process, very short electric pulses are applied to a colloidal ink solution that is sandwiched between a cathode electrode array and a passivated rotating electrode. The electrocoagulated ink adheres firmly to the positive electrode imaging cylinder. The adhered ink is transferable to plain paper after surplus ink has been removed. The ink is composed of a common linear, waste-water treatment polymer. The polymer is in suspension in water and forms a network that has a tendency to fold onto itself in the presence of metallic ions. The solvent is water mixed with electrolytic salts that render the ink electrically conductive.